This invention relates to an optical disk apparatus, and particularly to the improvement of the auto-focusing system of an optical disk apparatus which produces multiple recording/reproduction light spots with a single recording/reproduction head.
When an optical disk apparatus has its auto-focusing servo system started up and a light spot is focused on the disk surface, the amount of light returning from the optical disk to the laser source (semiconductor laser) increases suddenly and the intensity of laser emission also increases sharply due to the SCOOP (Self Coupled Optical Pickup) effect. The laser source is generally provided with an APC (Automatic Power Control) circuit for maintaining a constant laser output. However, the APC circuit with its long time-constant cannot respond to the above-mentioned sudden rise of emission and the laser intensity exceeds the recording threshold for a duration of several milliseconds, resulting possibly in the destruction of recorded data.
It is desirable to record and reproduce various types of optical disks having different characteristics with a single optical disk recording/reproduction apparatus. This requirement is dealt with by a known method in which control track information including the characteristics of each particular optical disk is recorded on the disk (generally on the innermost or outermost track) and it is read out at the operation, as disclosed in Japanese Patent Unexamined Publication No. 50-81695, for example.